Underdrains

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Underdrains comprise a length of perforated pipe embedded into a layer of reservoir gravel. They are a key component and vary hugely according to the drainage requirements of the installation, and the available maintenance access.

Infiltrating practices[edit]

The pipe within the drain should be elevated from the base to promote infiltration of the water stored beneath. The depth of this internal water storage should be sized according to the desired drainage time and the infiltration rate of the native soils below. An alternative design configuration permits the head of water to be stored by using an upturned outflow pipe.

  • At least one pair of vertical cleanout pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sediment. As most hydro-jetting apparatus used for this has some trouble accommodating narrow 90 deg bends, it is important that both ends of a perforated pipe be connected with a pair of 45 deg elbows/Y connectors instead.
Clean out spacing
Pipe internal diameter (mm) Maximum distance between cleanouts (m)
100 15
200 or greater 30

In a bioretention facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir gravel has a void ratio of 0.4, whilst most bioretention filter media may have a void ratio of 0.3 or lower. In some cases where the underdrain layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity. Ultimately this idea may result in the use of retention chambers to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatability of their systems with trees.

Non-infiltrating practices below ground[edit]

Below ground[edit]

Where a stormwater planter or biofiltration cell is contained within a concrete box or completely lined to prevent infiltration, the perforated pipe should be bedded on a thin layer of fine aggregate. This thin layer is to hold the pipe in place during construction, and to permit free ingress of accumulated water through holes on the underside of the pipe. As storage in a non-infiltrating practice is predominantly through soil/water tension, the depth of reservoir should be minimised to just accommodate the pipe. A pair of vertical clean out pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sediment. As most hydro-jetting apparatus used for this has some trouble accommodating narrow 90 deg bends, it is important that both ends of a perforated pipe be connected with a pair of 45 deg elbows/Y connectors instead.

Above ground[edit]

Where possible the underdrain pipe should be designed without any bends in order to facilitate easy maintenance. Otherwise see advice above regarding connectors.

  • To promote infiltration the base of the gravel reservoir and the underdrain pipe should be horizontal to optimize distribution of the water within.
  • Where drainage or conveyance to a downstream facility is a greater priority, the base of the reservoir and the underdrain pipe may have a gradient of up to 1-2%.


  • A minimum of 150 mm reservoir gravel should be laid beneath the perforated pipe.
  • A minimum of 300 mm reservoir gravel should be laid over the perforated pipe. An exception would be where a small bioretention installation is being made in a stormwater planter and no compaction of the reservoir material is undertaken. Otherwise this material is required to protect the pipe from the compaction processes.
  • A layer of 75 - 100 mm pea gravel may then be included on top of the reservoir gravel to provide a smoother surface and reduce tearing to the geotextile.
  • A layer of geotextile is usually then used to prevent migration of fines into the underdrain/reservoir space.

A pair of small wells are recommended to inspect and periodically flush accumulated sediment from the underdrain pipe.


Spacing drainage pipes[edit]

The yellow box represents the recommended hydraulic conductivity of bioretention filter media

In most LID underdrain applications, lateral drains should be spaced between 5 - 6 m apart.


This recommendation is supported by an analysis of Hooghoudt's equation [1][2][3] in relation to loamy or clayey native soils, where Kmedia>>Ksoil, finds the first term of the numerator negligible, so that the original equation: may be simplified to: Where: Kmedia is expressed in m/day Dd is the depth to the drain pipe (m) Dw is the minimum acceptable depth to the water table during infiltration event q is the inflow volume expressed as a depth over the entire surface (m)

Example[edit]

During a 25 mm storm event, a bioretention cell receives concentrated flow from a catchment 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m). The bioretention cell comprises 0.6 m filter media (K = 2.4 m/day), laid over 0.6 m clear, coarse reservoir gravel. The pipes are laid within the reservoir, 0.9 m below the surface. The system is designed to fill entirely during the rainstorm event. i.e. Depth to water table = 0 m.:

Recommendations for material specifications[edit]

Pipes[edit]

Pipes are available with perforations on just one side, these should be situated on the lower half of the pipe. Pipes with 360° perforations should have a strip of geotextile or membrane placed over the pipe to reduce the migration of fines from overlying media.

Perforated pipes are a common component of underdrains used in bioretention, permeable pavements, infiltration trenches and exfiltration systems.

Pipes should be manufactured in conformity with the latest standards by the Canadian Standards Association (CSA) or ASTM International.

  • Perforated pipes should be continuously perforated, smooth interior HDPE or PVC.
    • Wherever possible pipes should be ≥200 mm internal diameter to reduce potential of freezing and to facilitate push camera inspections and cleaning with jet nozzle equipment.
    • Smooth interior facilitates inspection and maintenance activities; internal corrugations can cause cameras or hydrojetting apparatus to become snagged.
    • A perforated pipe with many rectangular slots has better drainage characteristics than a pipe with similar open area provided by fewer circular holes [4].
  • Non-perforated pipes should be used for conveyance of stormwater to and from the facility, including overflow. It is good practice to extend the solid pipe approximately 300 mm within the reservoir or practice to reduce the potential for native soil migration into the pipe.

See also: Flow through perforated pipe


Reservoir gravel[edit]

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregate to be used to store water for infiltration. This is usually called 'clear stone' at aggregate yards.

To see an analysis of Ontario Standard Specifications for granular materials, see OPSS aggregates.

For advice on decorative surface aggregates see Stone


Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a porosity of 0.4[5].

The clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the porosity). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles [6]. See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

  • Minimum durability index of 35
  • Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregates are maintained by ASTM D2940


Pea gravel[edit]

Pea gravel

Geotextiles[edit]

The properties of geotextiles vary widely.

See Clogging for notes on their application in LID structures.

Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregates, and slump of heavier particles into finer underlying courses. Geotextiles are commonly used on low strength soils (CBR<4). The formation of biofilm on geotextiles has also been shown to improve water quality:

  • By degrading petroleum hydrocarbons[7]
  • By reducing organic pollutant and nutrient concentrations [8]
  • When installing geotextiles an overlap of 150 - 300 mm should be used.

Material specifications should conform to OPSS 1860 for Class II geotextile fabrics [9]. Note when expansive clays are present, a non-infiltrating design may be necessary. If used, geotextile socks around perforated pipes should conform to ASTM D6707 with minimum water flow rate conforming to ASTM D4491 (12,263 L/min/m2 at 5 cm head).

  • Fabrics should be woven monofilament or non-woven needle punched.
  • Woven slit film and non-woven heat bonded fabrics should not be used, as they are prone to clogging.

In choosing a product, consider:

  1. The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
  2. The load bearing ratio of the underlying native soil (i.e. is the geotextile needed to prevent downward migration of aggregate into the native soil?),
  3. The texture (i.e., grain size distribution) of the overlying and underlying materials, and
  4. The suitable apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, to maintain water flow even with sediment and microbial film build-up.
Recommended criteria for selection of geotextile fabric
Percent soil/filter media passing 0.075 mm (#200 sieve) Non-woven fabric apparent opening size (AOS, mm) Woven fabric percent open area (POA, %) Permittivity (sec-1)
>85 ≤ 0.3 - 0.1
50 - 85 ≤ 0.3 ≥ 4 0.1
15 - 50 ≤ 0.6 ≥ 4 0.2
5 - 15 ≤ 0.6 ≥ 4 0.5
≤ 5 ≤ 0.6 ≥ 10 0.5

Performance research[edit]

http://www.mdpi.com/2073-4441/7/4/1595/htm


Alternative Technology[edit]

[10]

  1. H.P.Ritzema, 1994, Subsurface flow to drains. Chapter 8 in: H.P.Ritzema (ed.), Drainage Principles and Applications, Publ. 16, pp. 236-304, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90-70754-33-9
  2. W.H. van der Molen en J.Wesseling, 1991. A solution in closed form and a series solution to replace the tables for the thickness of the equivalent layer in Hooghoudt's drain spacing equation. Agricultural Water Management 19, pp.1-16
  3. van Beers, W.F.J. 1976, COMPUTING DRAIN SPACINGS: A generalized method with special reference to sensitivity analysis and geo-hydrological investigations, International Institute for Land Reclamation and Improvement (ILRI) Wageningen, The Netherlands
  4. Hazenberg, G., and U. S. Panu (1991), Theoretical analysis of flow rate into perforated drain tubes, Water Resour. Res., 27(7), 1411–1418, doi:10.1029/91WR00779.
  5. Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017
  6. 6.0 6.1 6.2 Judge, Aaron, "Measurement of the Hydraulic Conductivity of Gravels Using a Laboratory Permeameter and Silty Sands Using Field Testing with Observation Wells" (2013). Dissertations. 746. http://scholarworks.umass.edu/open_access_dissertations/746
  7. Newman AP, Coupe SJ, Spicer GE, Lynch D, Robinson K. MAINTENANCE OF OIL-DEGRADING PERMEABLE PAVEMENTS: MICROBES, NUTRIENTS AND LONG-TERM WATER QUALITY PROVISION. https://www.icpi.org/sites/default/files/techpapers/1309.pdf. Accessed July 17, 2017.
  8. Paul P, Tota-Maharaj K. Laboratory Studies on Granular Filters and Their Relationship to Geotextiles for Stormwater Pollutant Reduction. Water. 2015;7(4):1595-1609. doi:10.3390/w7041595.
  9. ONTARIO PROVINCIAL STANDARD SPECIFICATION METRIC OPSS 1860 MATERIAL SPECIFICATION FOR GEOTEXTILES. 2012. http://www.raqsb.mto.gov.on.ca/techpubs/OPS.nsf/0/2ccb9847eb6c56738525808200628de1/$FILE/OPSS%201860%20Apr12.pdf. Accessed July 17, 2017
  10. Redahegn Sileshi; Robert Pitt, P.E., M.ASCE; and Shirley Clark, P.E., M.ASCEPerformance Evaluation of an Alternative Underdrain Material for Stormwater Biofiltration Systems, Journal of Sustainable Water in the Built Environment, 4(2), May 2018 https://doi.org/10.1061/JSWBAY.0000845